a) Field of the invention
The present invention relates to a new organic friction material composition useful to produce brake linings, clutch linings and, more generally, any kind of friction linings that must be capable of withstanding high operating temperatures and pressures for substantial periods of time.
The invention also relates to friction linings, including brake linings and clutch linings, obtained from such a composition, especially by wet or dry molding.
b) Brief description of the prior art
It is of common practice to use compositions containing organic or inorganic fibers dispersed in a thermosetting resin to produce friction linings and more particularly brake linings for use in cars, trucks, heavy duty machineries and the like. These compositions that are usually called "organic friction material compositions", may also contain friction additives such as cashew nut shell powder or oil, copper or brass particles, coal or coal derivatives, and the like, in such an amount as to adjust the friction and the properties of the material to some predetermined values. These compositions may further comprise other additives commonly used in any molding compositions, such as graphite which is known to be a molding lubricant. These compositions may further contain low-cost fillers such as baryte, calcium carbonate, chromite, iron oxide, natural or synthetic rubber, talc, wollastonite and similar materials, whose purpose is exclusively to reduce the amount of resin being used and thus the total cost of the linings made from such compositions. They may further contain natural organic fibers such as cellulose, synthetic organic fibers such as KEVLAR (trade mark), mineral fibers such as mineral wool or glass fibers, or metallic fibers such as steel fibers, for reinforcing purposes.
In practise, the friction linings that are obtained from the above mentioned compositions, may be classified in two broad categories, depending on the way they are produced.
The first category includes the friction linings obtained by impregnation of a fabric made of woven fibers. According to this process, a fabric made of woven asbestos fibers is reinforced by a wire of brass or copper and is impregnated with a thermosetting resin of the china oil or linseed oil type, that is subsequently polymerized. Use can also be made of sodium silicate instead of the resin.
The friction linings that are so-produced are often used in brakes or clutches in which the linings are immersed into a liquid, such as oil (wet applications). Examples of such applications are into public transportation vehicles, military vehicles, industrial vehicles or farm equipments.
The other category includes all the friction linings that are obtained by dry or wet molding.
Usually, the compositions used for the manufacture of friction linings by dry molding comprises the following four major components:
1- a thermosetting resin which is usually a dry resin of the phenolic type;
2- reinforcing fibers which usually consist of chrysotile asbestos;
3- friction additives suitably selected to provide the requested friction properties; and
4- low-cost fillers to reduce the total cost of the linings.
The dry-molding process comprises the following steps.
1- Dry mixing of all the components in a mixer until a good homogeneity and an adequate bulk density are achieved.
2- Cold pressure molding of the mixture into preformed shapes to further reduce the bulk density of the mixture. Such preforms must have some preselected characteristics of hardness, firmness and flexural strength to pass through the subsequent steps.
3- Hot molding and final curing. The preforms are introduced into a mold and compressed to a pressure of about 2000 psi at a temperature of 125.degree. to 135.degree. C. for a period of time of about 4 minutes. The final curing is carried out in an oven for about 2 hours at 125.degree. C.
To be properly molded, any composition must have good flow properties; it must not lead to the formation of blisters and not stick to the wall of the mold. All of these properties allow for proper qualification of a composition according to its ability to be molded or manufactured industrially. Such properties however are not sufficient to determine whether the composition will result in linings having good mechanical properties and sufficient friction properties, such properties also depending on the relative amount of the basic components of the composition.
Friction linings produced by dry-molding as disclosed hereinabove are often used as disc pad brake linings for typical application in cars, or as block brake linings.
The composition that are used for the manufacture of friction linings by wet-molding comprises the same four major components as disclosed hereinabove, except for the thermosetting resin which is liquid and may be used with or without solvent.
The wet molding process used to produce friction linings from such liquid composition consists in mixing the components in a mixer such as a SIGMA.RTM. mixer, then forming the mixture into a calendering machine comprising two rollers and subjecting the so-formed mixture to hot-polymerization under pressure.
Friction linings produced by wet-molding are generally used as roll brake linings in passenger cars.
In the past, the fiber that has widely been used to prepare organic friction material compositions whatever be their "categories" is asbestos, because of its very low cost and its ability to provide friction linings having good durability and good friction and strength properties.
Over the last decade, other fibers have been proposed as substitutes for asbestos to manufacture organic friction material compositions. Examples of such "alternative" compositions are disclosed by way of examples in U.S. Pat. No. 4,119,591 and No. 4,461,643, and in the other prior art references that are mentioned in the preamble of these two patents.
U.S. patent application Ser. No. 246,198 filed on Nov. 8, 1988 in the name of the same Applicant, discloses and claims a fibrous-like synthetic forsterite product which is particularly useful as an insulating material. This product which is presently offered for sale under the trademark FRITMAG and will be called as such hereinafter, is obtained by subjecting chrysotile asbestos fibers of any commercial grade, having an MgO: SiO2 ratio lower than 1:1, to calcination at a temperature of from 650.degree. to 1450.degree. C.
FRITMAG has a raw loose density of from 3 to 40 pounds per cubic foot, a thermal conductivity K factor of from 0.25 to 0.40 BTU. in/hr. .degree.F.ft.sup.2 and a fusion point of about 1600.degree. to 1700.degree. C. It possesses a somewhat fibrous structure resembling that of the chrysotile asbestos fibers from which it derives, although this fibrous structure has shown to disappear upon rough manipulation, when subjected to pressure, or when mixed with other material. Then, the fibrous structure is lost but the product has and always retains a high insulating value which is quite superior to granular forsterite, or similar to KAOWOOL (trademark) or rockwool.
In the above-mentioned U.S. patent application, it is mentioned that FRITMAG may be used as a substitute for asbestos, whenever a fibrous material to be used in bulk and having high insulating qualities is needed. Indeed, FRITMAG has a loose density range substantially identical to asbestos. It also has high insulating properties and is devoided of all the undesirable health problems allegedly attributed to asbestos. In particular, the above-mentioned U.S. patent application suggests to mix FRITMAG with an inert filler and a binder in order to form an insulating composition adapted to be shooted onto any surface to be insulated or to be moulded in the form of slabs for roof insulation. However, no specific example of such a composition is given, and no other use is suggested, except for a very short reference made in the specification to the possibility of mixing FRITMAG with other materials, such as Portland cement.